Mold construction forming single crystal pieces



June 2 1970 3,515,205

MOLD CONSTRUCTION FORMING SINGLE CRYSTAL PIECES C. M. WICKSTRAND, JR

Filed March 20, 1968 United States Patent U.S. Cl. 164-353 7 ClaimsABSTRACT OF THE DISCLOSURE A mold construction for use in the formationof single crystal pieces of relatively long lengths or of complex shapesand wherein the mold geometry provides a means for increasing the rateof solidification within a plurality of individual molds by providing aneflicient method of heat extraction from the mold.

BACKGROUND OF THE INVENTION This invention relates to a moldconstruction for the formation of single crystal pieces or stock.

In casting single crystal stock it becomes extremely desirable if thesingle crystal stock or pieces can be cast in relatively large or widepieces. A problem heretofore which prevents producing relatively largeor wide single crystal stock is that the geometry of conventional molds,for producing single crystal stock has caused the heat extraction ratefrom the mold to be less than desirable. The problem with current singlecrystal molds is that these molds in promoting single crystal growth usea double bend, or pigtail shape restriction at the lower end of the moldAs a result of this restriction, heat from the mold does not have aready path to the chill plate. Consequently, the heat extraction rate islower than required for the production of large or wide single crystalstock. It therefore becomes clear that to produce relatively large orwide single crystal stock it is necessary to provide a moldconstruction, the geometry of which will provide a more eflicient heatextraction rate than current molds while simultaneously promoting thefastest growth rate consistent with single crystal growth.

Additionally, as a result of the geometry of current molds, the lowerportions of the molds are susceptible to damage or contamination byexplosions that occur when the molten metal first contacts thecontaminated surface of the water cooled chill plate. The presentinvention avoids this problem and further provides a mold constructionthe geometry of which facilitates removal of the single crystal molds.

SUMMARY OF THE INVENTION It is a primary object of this invention toprovide a novel mold construction wherein the rate of solidificationwithin a plurality of individual molds is increased by virtue of a moreeflicient means of heat extraction therefrom. Additionally, this rate iscompatible with a rate which promotes the fastest growth consistent withsingle crystal growth.

To accomplish the foregoing, the present invention uses the well-knownprocess of unidirectional solidification in conjunction with a novelmold geometry. In this construction, a ceramic tube is placed on a chillplate and around the inner periphery or in an annular chamber formed bytwo ceramic tubes is placed a plurality of individual molds. Theindividual molds may be of either a complex shape such as a vane orblade or of a single shape such as a wire, bar or rod. Each of theseindividual molds contains a small opening, normally positioned at thebottom of each mold. The bottoms of these molds ice are positioned abovethe chill plate, a preferred distance being one-half inch. When thedirectional solidification process begins, and the liquid-metalinterface moves away from the chill plate, it has a particular columnaroriented structure. Therefore, when the liquid metal interface reachesthe small openings of the individual molds, single crystal growth willbe promoted therein, the axis of orientation of the single crystalgrowth being a function of the axis of orientation of the directionallysolidified metal, the orientation of the opening with each mold and/orthe orientation of the mold itself.

By employing the type of mold geometry herein described, it becomesobvious that the only restriction to heat transfer from the molten metalexists in the individual molds. More specifically, the small opening ineach mold is a restriction to heat transfer from the metal in the moldto the chill plate. It is pointed out that the restriction as describedherein is a significant improvement over current mold designs that use adouble or pigtail opening in the mold and this alone would provide amore efiicient or increased extraction of heat from the mold. However,the mold geometry of the present invention provides a further increasein the extraction of heat from the individual molds. Since theaforementioned opening is a restriction to the transfer of heat from themold to the chill plate, the liquid metal interface within theindividual molds moves at a slower rate than the liquid metal interfacewithin the ceramic tube. Significantly, the volume within this tube andhence the mass of the molten metal therewithin is substantially greaterthan the mass Within the individual molds. Since the liquid metalinterface of this mass moves more quickly than that within theindividual molds. it is consequently at a lower temperature. Therefore,heat from the individual molds is transferred to the mass within thetube. Since this mass is substantial with respect to the mass within theindividual molds, a more efficient rate of heat extraction is possible.In summary, it is pointed out that the mold geometry of the presentinvention provides a more efiicient means for extracting heat from asingle crystal mold by providing a minimum of restriction to heattransfer from the mold to the chill plate and by providing an additionalheat path for the transfer of heat from the individual mold to thesubstantial mass surrounding the individual molds.

Additionally, the mold geometry herein described permits easy removal ofthe individual molds 'while preserving the ceramic tube. Thisremovabilit-y feature is enhanced by placing a layer of ceramic aroundeach of the individual molds.

Further, it should be understood that the individual molds of thepresent invention can be arranged in any configuration within and aroundthe tube. Similarly, the individual molds can be of any shape desired,such as a. simple shape, a complex shape, or a combination of both.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic top sectionalview of the mold and mold geometry of the present invention.

FIG. 2 is a vertical section of a mold in accordance with the presentinvention.

FIG. 3 is a schematic top sectional view showing a modified embodimentof a mold geometry in accordance with the present invention.

FIG. 4 is a vertical section of the modified mold in accordance with thepresent invention.

FIG. 5 is an enlarged view of the circled area of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now in detail to thepresent preferred embodiments of the present invention, a novel form ofmold geometry is shown in FIGS. 1 and 2. The mold construction describedherein is particularly suited for use with any of the so-called superalloys as described for example in the Ver Snyder Pat. US. No.3,260,505, and having the same assignee as this application. As thereinnoted, these alloys are generally adapted for the process known asdirectional solidification. The mold construction herein described, inaddition to the disclosure contained in the Ver Snyder patent, employsthe technique of forming Monocrystaloys' as described in an applicationto Piearcey, U.S. Ser. No. 540,114 filed Feb. 17, 1966, and assigned tothe same assignee as this application.

As herein illustrated, one end of tubular mold 4, compatible for usewith the procedure described in the Ver Snyder patent, is placed on arelatively cool, heat conductive and preferably water cooled plate 6.Tubular mold is preferably made from a ceramic material from aconventional slurry of alumina or other high melting point refractorymaterial in accordance with standard shellmolding techniques; and, waterfor the chill plate 6 is carried through conduits 8. As illustrated, oneend of tube 4 rests on chill plate 6 and cooperates to form an enclosedcavity 10. The end of tube 4 opposite chill plate 6 is open to receivemolten metal.

Surrounding cavity 10 are the means for heating the mold to the desiredtemperature for casting. Preferably, the cavity is surrounded by agraphite susceptor, not shown, and this in turn is surrounded by aninduction coil 12 supplied with high frequency electric current as isusual in a high frequency induction furnace. Prior to casting, the moldis heated to a desired temperature by supplying current to coil 12 andwhen the desired temperature has been attained, molten metal, heated tothe proper temperature for casting, is poured into cavity 10. The chillplate 6 is maintained at a relatively cool temperature by means of watercirculating through conduits 8 so as to establish a temperature gradientwithin the molten metal filling cavity 10 as the metal solidifies.

In the present embodiment, a plurality of individual molds 16, 17, 18,19, 20, 22 are positioned around the inner periphery or surface 23 oftube 4. Each of these molds has a small opening 24, herein illustratedto be at the bottom of each mold, and is spaced above chill plate 6. Nowas the metal begins to solidify it has a controlled columnar structureand by providing a small opening such as 24 in each of the individualmolds 16, 17, 18, 19, 20 and 22, growth of a single crystal is promotedtherein. Therefore, What is produced is in effect a large mass ofcontrolled columnar structure surrounding relatively smaller masses ofsingle crystal. A significant feature about the construction of theindividual molds is that the small opening 24 in each of the molds isthe only restriction to heat transfer from each of the molds to chillplate 6. As a result of this, this type mold geometry would permit theheat within the mold to be extracted at a faster rate than the currentmold constructions which use a double restriction or pigtail shape asillustrated in the Piearcey application Ser. No. 540,114 filed Feb. 17,1966.

As hereinbefore noted, the volume of tube 4 is substantially greaterthan the volume within each of molds 16, 17, 18, 19, 20, and 22.Therefore, a relatively greater mass surrounds molds 16, 17, 18, 19, 20and 22 than is interior thereof. Now as the directional solidificationproceeds, the liquid-to-solid interface moves away from chill plate 6.As a result of the restriction to heat transfer from the individualmolds 16, 17, 18, 19, 20 and 22 to chill plate 6 caused by opening 24,the liquid solid interface interior of molds 16, 17, 18, 19, 20 and 22is slower moving than that of the mass surrounding the molds. In otherwords, any distance from the chill plate, the solidification of thesubstantial mass within the tube will be higher than the solidificationwithin molds 16, 17, 18, 19, 20 and 22. This also means that the masssurrounding the molds is at a lower temperature, and heat will beextracted laterally from molds 16, 17, 18, 19, 20 and 22.

4 By removing heat from molds 16, 17, 18, 19, 20 and 22 at thisincreased or more efficient rate faster single crystal growth ispromoted within the molds and relatively larger and more complex shapescan be produced.

To assist in removing individual molds 16, 17, 18, 20 and 22, each ofthe molds is surrounded by a layer of ceramic material 26. It should beunderstood that layer 26 performs other functions, such as assistingheat extraction from the mold.

The mold construction illustrated in FIGS. 3 and 4, and 5 is anotherembodiment of the present invention and it has as an advantage the factthat it does not waste a significant amount of molten metal. As showntherein, the mold consists of an inner tube 102 and an outer tube 104forming an annular chamber 106 therebetween. The tubes 102 and 104 areof the same construction as tube 4 of FIG. 1 and are similarly placed onchill plate 108. The individual molds 112, 114, 118, and 122 in thisembodiment are placed in annular chamber 106 and molten metal is pouredonly in annular chamber 106. The single crystal pieces are formed inindividual molds 112, 114, 116, 118, 120 and 122 in the same manner asFIGS. 1 and 2, the single crystal entering the molds through opening124.

It is pointed out that this type configuration can be employed in theindividual molds. More specifically, once the single crystal growth ispromoted within the mold, the metal can be caused to grow through acentral core 134 and around core 134 to the desired shape 136. This typeindividual mold configuration further increases the rate of heatextraction. Also as in FIGS. 1 and 2, the individual molds aresurrounded by a layer of ceramic material 126.

In each of FIGS. 1, 2 and 3 the individual molds are shown positionedabove the chill plate. This is of importance in that it has been foundthat during the directional solidification process, the solidified metaladjacent the chill plate has a random crystal orientation. Accordingly,to get a single crystal with a preferred orientation, the individualmolds should be positioned above the chill plate a distance sufficientto avoid this growth zone. It is clear that individual molds may bepositioned at the same height above the chill plate or they may beplaced in a stepped arrangement thereabove. However, it has been foundthat an optimum spacing from the chill plate to the lowest individualmold should be about one-half inch.

It is to be understood that the invention is not limited to theembodiments herein illustrated and described but may be used in otherways without departure from its spirit as defined by the followingclaims.

What is claimed is:

1. A mold construction for producing single crystal pieces comprising:

a ceramic tube having an open end;

a chill plate, said open end resting on the chill plate plate and theremainder of the tube being open to receive molten metal; and

a plurality of individual elongated molds arranged around the innerperiphery of the tube and spaced from the end of the tube resting on thechill plate, each of said molds having bottom means forming a relativelysmall opening to permit the entrance of a single grain of the moltenmetal, said means being the only restriction in each of the molds to thetransfer of heat from the molds to the chill plate and the volume withineach of said molds being substantially less than the volume within thetube so that when molten metal is poured into the tube the mass thereinis substantially greater than the mass interior of each mold.

2. A mold construction as in claim 1 wherein:

the opening in each mold is at least one-half inch from the end of thetube resting on the chill plate.

3. A construction as in claim 1 wherein:

the openings in the individual mold are spaced in six steppedrelationship above the end of the tube resting on the chill plate, theleast spacing being at least onehalf inch.

4. A mold construction for producing single crystal pieces, comprising:

a first ceramic tube having an open end;

a second ceramic tube having an open end, the first and second tubesbeing positioned to form an annular chamber therebetween;

a chill plate, the open end of the first tube and the open end of thesecond tube resting thereon, the remainder of the first and second tubesbeing open to receive molten metal; and

a plurality of individual elongated molds positioned within said annularchamber and spaced from the end of the tubes resting on the chill plate,each of said molds having bottom means forming a relatively smallopening to permit the entrance of a single grain of the molten metal,the means being the only restriction to the transfer of heat from eachmold to the chill plate and the volume within the individual molds beingsubstantially less than the volume within the annuar chamber so thatwhen molten metal is poured into the annular chamber, the mass thereinis substantially greater than the mass interior of each mold.

5. A mold construction as in claim 4 wherein:

the molds have a central core into which molten metal enters so that theouter surface and the inner surface of the core is surrounded by moltenmetal.

6. A mold construction as in claim 4 wherein:

the opening in each mold is at least one-half inch above the end of thetubes resting on the chill plate.

7. A mold construction as in claim 4 wherein:

the individual molds are surrounded by a layer of ceramic material.

References Cited UNITED STATES PATENTS 2,654,925 10/1953 Ensign et a1164-363 X 3,417,809 12/1968 Sink 164-353 X FOREIGN PATENTS 458 2/1886Sweden.

ROBERT D. BALDWIN, Primary Examiner U.S. Cl. X.R.

